The present invention relates generally to medical devices and methods and, more particularly, to a programmable, microprocessor based controller and method for controlling the temperature and flow of a thermal exchange fluid that is circulated through a heat exchange catheter inserted into a patient""s body for the purpose or cooling or warming at least a portion of the patient""s body.
Under ordinary circumstances, the thermoregulatory mechanisms of a healthy human body serve to maintain the body at a constant temperature of about 37xc2x0 C. (98.6xc2x0 F.), a condition sometimes referred to as normothermia. To maintain normothermia, the thermoregulatory mechanisms act so that heat lost from the person""s body is replaced by the same amount of heat generated by metabolic activity within the body. For various reasons such as extreme environmental exposure to a cold environment or loss of thermoregulatory ability as a result of disease or anesthesia, a person may develop a body temperature that is below normal, a condition known as hypothermia. A person may develop a condition that is above normothermia, a condition known as hyperthermia, as a result of extreme exposure to a hot environment, or malfunctioning thermoregulatory mechanisms, the latter being a condition sometimes called malignant hyperthermia. The body may also establish a set point temperature (that is, the temperature which the body""s thermoregulatory mechanisms function to maintain) that is above normothermia, a condition usually referred to as fever. The present invention addresses all of these situations.
Accidental hypothermia is generally a dangerous condition that may even be life threatening, and requires treatment. If severe, for example where the body temperature drops below 30xc2x0 C., hypothermia may have serious consequences such as cardiac arrhythmias, inability of the blood to clot normally, or interference with normal metabolism. If the period of hypothermia is extensive, the patient may even experience impaired immune response and increased incidence of infection.
Simple methods for treating accidental hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. If the hypothermia is not too severe, these methods may be effective. However, wrapping a patient in a blanket depends on the ability of the patient""s own body to generate heat to re-warm the body. Administering warm fluids by mouth relies on the patient""s ability,to swallow, and is limited in the temperature of the liquid consumed and the amount of fluid that may be administered in a limited period of time. Immersing a patient in warm water is often impractical, particularly if the patient is simultaneously undergoing surgery or some other medical procedure.
More recently, hypothermia may be treated in a more complex fashion. Heated warming blankets may be applied to a patient or warming lamps that apply heat to the skin of the patient may be used. Heat applied to the patient""s skin, however, has to transmit through the skin by conduction or radiation which may be slow and inefficient, and the blood flow to the skin may be shut down by the body""s thermoregulatory response, and thus, even if the skin is warmed, such mechanisms may be ineffective in providing heat to the core of the patient""s body. When breathing gases are administered to a patient, for example a patient under anesthesia, the breathing gases may be warmed. This provides heat relatively fast to the patient, but the amount of heat that can be administered without injuring the patient""s lungs is very limited. An alternative method of warming a hypothermic patient involves infusing a hot liquid into the patient via an IV infusion, but this is limited by the amount of liquid that can be infused and the temperature of the liquid.
In extreme situations, a very invasive method may be employed to control hypothermia. Shunts may be placed into the patient to direct blood from the patient through an external machine such as a cardiopulmonary by-pass (CPB) machine which includes a heater. In this way, the blood may be removed from the patient, heated externally, and pumped back into the patient. Such extreme measures have obvious advantages as to effectiveness, but also obvious drawbacks as to invasiveness. The pumping of blood through an external circuit that treats the blood is generally quite damaging to the blood, and the procedure is only possible in a hospital setting with highly trained personnel operating the equipment.
Accidental hyperthermia may also result from various conditions. Where the normal thermoregulatory ability of the body is lost, because of disease or anesthesia, run-away hyperthermia, also known as malignant hyperthermia, may result. The body may also set a higher than normal set point resulting in fever which is a type of hyperthermia. Like hypothermia, accidental hyperthermia is a serious condition that may sometimes be fatal. In particular, hyperthermia has been found to be neurodestructive, both in itself or in conjunction with other health problems such as traumatic brain injury or stroke, where a body temperature in excess of normal has been shown to result in dramatically worse outcomes, even death.
As with hypothermia, when the condition is not too severe, simple methods for treating the condition exist, such as cold water baths and cooling blankets, or antipyretic drugs such as aspirin or acetominophen, and for the more extreme cases, more effective but complex and invasive means such as cooled breathing gases, cold infusions, and blood cooled during CPB also exist. These, however, are subject to the limitations and complications as described above in connection with hypothermia.
Although both hypothermia and hyperthermia may be harmful and require treatment in some case, in other cases hyperthermia, and especially hypothermia, may be therapeutic or otherwise advantageous, and therefore may be intentionally induced. For example, periods of cardiac arrest or cardiac insufficiency in heart surgery result in insufficient blood to the brain and spinal cord, and thus can produce brain damage or other nerve damage. Hypothermia is recognized in the medical community as an accepted neuroprotectant and therefore a patient is often kept in a state of induced hypothermia. Hypothermia also has similar advantageous protective ability for treating or minimizing the adverse effects of certain neurological diseases or disorders such as head trauma, spinal trauma and hemorrhagic or ischemic stroke. Therefore it is sometimes desirable to induce whole-body or regional hypothermia for the purpose of facilitating or minimizing adverse effects of certain surgical or interventional procedures such as open heart surgery, aneurysm repair surgeries, endovascular aneurysm repair procedures, spinal surgeries, or other surgeries where blood flow to the brain, spinal cord or vital organs may be interrupted or compromised. Hypothermia has even been found to be advantageous to protect cardiac muscle tissue after a myocardial infarct (MI).
Current methods of attempting to induce hypothermia generally involve constant surface cooling, by cooling blanket or by alcohol or ice water rubs. However, such cooling methods are extremely cumbersome, and generally ineffective to cool the body""s core. The body""s response to cold alcohol or ice water applied to the surface is to shut down the circulation of blood through the capillary beds, and to the surface of the body generally, and thus to prevent the cold surface from cooling the core. If the surface cooling works at all, it does so very slowly. There is also an inability to precisely control the temperature of the patient by this method.
If the patient is in a surgical setting, the patient may be anesthetized and cooled by CPB as described above. Generally, however, this is only available in the most extreme situations involving a full surgical team and full surgical suite, and importantly, is only available for a short period of time because of the damage to the blood caused by pumping. Generally surgeons do not wish to pump the blood for periods longer than 4 hours, and in the case of stroke or traumatic brain damage, it may be desirable to induce hypothermia for longer than a full day. Because of the direct control of the temperature of a large amount of blood, this method allows fairly precise control of the patient""s temperature. However, it is this very external manipulation of large amounts of the patient""s blood that makes long term use of this procedure very undesirable.
Means for effectively adding heat to the core of the body that do not involve pumping the blood with an external, mechanical pump have been suggested. For example, a method of treating hypothermia or hyperthermia by means of a heat exchange catheter placed in the bloodstream of a patient was described in U.S. Pat. No. 5,486,208 to Ginsburg, the complete disclosure of which is incorporated herein by reference. Means of controlling the temperature of a patient by controlling such a system is disclosed in U.S. Pat. No. 5,837,003, also to Ginsburg, the complete disclosure of which is incorporated herein by reference. A further system for such controlled intervascular temperature control is disclosed in publication WO 00/10494 to Ginsburg et al., the complete disclosure of which is incorporated herein by reference. Those patents and publication disclose a method of treating or inducing hypothermia by inserting a heat exchange catheter having a heat exchange area including a balloon with heat exchange fins into the bloodstream of a patient, and circulating heat exchange fluid through the balloon while the balloon is in contact with the blood to add or remove heat from the bloodstream. (As used herein, a balloon is a structure that is readily inflated under pressure and collapsed under vacuum.)
A number of catheter systems for cooling tissue adjacent the catheter or regulating the temperature of the catheter using the temperature of fluid circulating within the catheter are shown in the published art. Some such catheters rely on a reservoir or similar tank for a supply of heat exchange fluid. For example, U.S. Pat. No. 3,425,419 to Dato, U.S. Pat. No. 5,423,811 to Imran et al., U.S. Pat. No. 5,733,319 to Neilson, et al., U.S. Pat. No. 6,019,783 to Phillips, et al., and. U.S. Pat. No. 5,624,392 to Saab disclose catheters with circulating heat exchange fluid from a tank or reservoir. For such systems that involve a catheter placed in the bloodstream, however, difficulties arise in sterilizing the fluid source between uses and rapidly changing the temperature of a large volume of fluid having a significant thermal mass.
For the foregoing reasons, there is a need for a rapid and effective means to add or remove heat from the fluid supply for a catheter used to control the body temperature of a patient in an effective and efficient manner, while avoiding the inadequacies of the prior art methods. In particular, a fluid source that rapidly, efficiently and controllably regulates a disposable source of fluid based on feedback from the temperature of the patient or target tissue within the patient would be a great advantage.
The present invention avoids many of the problems of the prior art by providing an improved system to control the heating and/or cooling of a catheter with a body. The system generally includes a control unit exterior to body, a number of conduits extending from the control unit, and a heat transfer catheter in communication with the control unit via the conduits. The control unit modulates the temperature of a heat transfer region on the catheter using an advantageous control methodology to avoid over-shooting a target temperature. The catheter and conduits preferably define a fluid circulation path, wherein the control unit modulates the temperature of the heat transfer region by adjusting the temperature of a heat transfer fluid within the circulation path. Desirably, the control unit defines a cavity and the conduits are connected to a cassette that fits within the cavity, the cassette having an external heat exchanger through which the heat exchange fluid flows.
In one aspect of the present invention, a controller for controlling the temperature and flow of heat exchange fluid within a circuit is provided. The circuit is of a type that includes a heat exchange catheter, an external heat exchanger, and a pump for flowing heat exchange fluid through the circuit. The controller includes a heat and/or cold generating element in thermal contact with the external heat exchanger containing the heat exchange fluid. A patient sensor is positioned and configured to generate a signal representing a biophysical condition of the patient. The microprocessor in the controller receives the signal from the patient sensor and responds by controlling the generating element. The control unit further includes a mechanical drive unit for activating the pump contained in the circuit, and a safety sensor for detecting a fluid parameter in the circuit to generate a safety signal representative of the presence or absence of the fluid parameter. The safety signal is transmitted to the microprocessor that responds by controlling the operation of the pump. The sensor may be a bubble detector, and the fluid parameter is gas entrained in the heat exchange fluid. Alternatively, the circuit further comprises a reservoir, and the sensor is a fluid level detector for detecting a low fluid level in the reservoir.
In a still further aspect of the present invention, a heat transfer catheter flow system comprises a heat transfer medium circulation loop including a transfer catheter, a heat transfer unit, and conduits coupled to the heat transfer catheter and heat transfer unit that enable circulation of the heat transfer medium therebetween. The system further includes a pump head in contact with heat transfer medium within the circulation loop for circulating the medium through the loop. A cassette including a heat transfer unit and the pump head mates with a controller housing a control circuit and a pump motor so that the pump head engages the pump motor. An electronic feedback loop that detects back-torque experienced by the pump motor provides feedback to a control circuit that in turn controls the speed of the pump motor.
In another aspect, the present invention provides a controller for controlling the temperature and flow of heat exchange fluid within a circuit of the type that has a heat exchange catheter, an external heat exchanger, and a pump for flowing heat exchange fluid through the circuit. The controller includes a heat and/or cold generating element in thermal contact with the external heat exchanger. A mechanical drive unit activates the pump contained in the circuit to pump the heat exchange fluid. The controller includes a microprocessor connected to control both the generating element and the mechanical drive unit. A safety system is provided for detecting problems in the circuit. The safety system includes a plurality of sensors that generate signals indicative of respective parameters of the system and/or patient. The signals are transmitted to the microprocessor that responds by controlling the operation of the generating element and the mechanical drive unit. In one embodiment, the safety system includes a sensor for detecting the fluid level within the circuit. In a further embodiment, the safety system includes a sensor for detecting the temperature of a location within the patient, and further may include a redundant sensor for detecting the temperature of a location within the patient wherein a microprocessor is responsive to a difference in the two sensed patient temperatures. Furthermore, the safety system may include sensors for detecting bubbles within the circuit, detecting the operating status of the generating element, or detecting the operating status of the mechanical drive unit.
In one embodiment of the invention, a heat transfer catheter system includes a heat transfer catheter, a heat transfer unit, and conduits coupling the two elements and enabling circulation of heat transfer medium therebetween. The heat transfer unit defines a flow channel between opposite sidewalls, one of the sidewalls being relatively thin and flexible and providing minimal thermal insulation, while the opposite sidewall is relatively non-flexible so as to provide structural support to the heat transfer unit. The system may include a controller having a cavity for receiving the heat transfer unit and a heat and/or cold generating element therein positioned adjacent the flexible sidewall when the heat transfer unit is inserted within the cavity. The cavity may be sized such that outward expansion of the flexible sidewall upon flow of heat exchange medium through the flow channel causes the heat transfer unit to become compressively retained within the cavity. Desirably, the flexible sidewall attaches to the opposite sidewall both around their respective edges and along a series of lines within the edges such that the flow channel defines a serpentine path.
The present invention also provides a method of regulating the temperature of patient, comprising the steps of:
providing a heat exchange catheter system including a heat exchange catheter having a fluid path therethrough, a pair of conduits fluidly connected to the heat exchange catheter, and an external heat exchanger connected via the conduits to circulate heat exchange medium through the exchange catheter;
providing a first controller adapted to couple to the external heat exchanger of the heat exchange catheter system, the first controller including a heat and/or cold generating element therein for exchanging heat at a first rate with the heat exchange medium within the external heat exchanger;
providing a second controller adapted to couple to the external heat exchanger of the heat exchange catheter system, the second controller including a heat and/or cold generating element therein for exchanging heat at a second rate with the heat exchange medium within the external heat exchanger;
coupling the heat exchange catheter system with the first controller;
inserting the heat exchange catheter into the patient;
regulating the temperature of the patient by exchanging heat at the first rate between the generating element of the first controller and the external heat exchanger;
de-coupling the heat exchange catheter system from the first controller;
coupling the heat exchange catheter system with the second controller; and
regulating the temperature of the patient by exchanging heat at the second rate between the generating element of the second controller and the external heat exchanger.
The method may include performing a therapeutic or diagnostic procedure on the patient between the steps of de-coupling the heat exchange catheter system from the first controller and the step of coupling the heat exchange catheter system with the second controller. Indeed, the first controller and the second controller may be the same physical device.
In a still further method of the present invention, the rate of change of a patient""s body temperature is controlled using a heat transfer catheter and associated controller. The transfer catheter has a heat transfer region thereon, and the controller is placed in communication with the catheter via conduits. The controller is adapted to elevate or depress the temperature of the catheter heat transfer region relative to the body temperature. The patient""s body temperature within a body cavity or in another location is sensed, while the temperature of the heat transfer region is determined. A target temperature is then selected. The target temperature may be different than the body temperature, or may be the same if maintenance of normal patient temperature is the goal. A ramp rate equal to the time rate of change of temperature from the body temperature to the target temperature is selected. The temperature of the transfer region of the catheter based on the ramp rate is set. The method includes monitoring the temperature differential between the target temperature and the body temperature, and reducing the ramp rate when the temperature differential reduces below a predetermined threshold. Desirably, the heat transfer catheter and conduits defined a fluid circulation path therethrough, wherein the step of setting the temperature of the catheter heat transfer region comprises setting the temperature of a circulating fluid within the circulation path. Preferably, the step of determining the temperature of the catheter heat transfer region comprises directly or indirectly sensing the temperature of the circulating fluid. A comparison may be made between the target temperature and the temperature of the circulating fluid, which is then used to adjust the temperature of the circulating fluid.
In one aspect of the invention, the reservoir section is provided with a means to detect the fluid level in the reservoir and comprises at least one prism mounted within the reservoir section adjacent the inside of a relatively transparent window or wall portion in the reservoir, and at least one optical beam source and at least one optical beam sensor mounted on the reusable control unit adjacent the outside of the window. In one specific embodiment, the fluid level detector comprises a prism mounted in the reservoir, a light beam source and a light beam sensor. The prism has a diffraction surface and the light beam source directs a light beam against that surface. The prism is configured so that when the diffraction surface is in contact with air, the light beam is reflected to impinge on the light beam sensor and the sensor generates a signal. Likewise, when the diffraction surface is in contact with fluid, the light beam does not reflect to the sensor and the sensor does not generate a signal.
In operation, a light beam is directed through the reservoir section and against the prism at a particular point along its angled length. The sensor is located to detect the presence or absence of a reflected beam. As long as the fluid reservoir remains full and the fluid level is at a pre-determined elevation above the point of impingement of the light beam, the diffraction surface of the prism at that point is in contact with the fluid. Therefore, the light beam directed at the prism travels through the prism and, upon reaching the diffraction surface, is reflected such that the sensor does not observe a reflected beam. If the fluid falls below the pre-determined elevation, the diffraction surface of the prism at the point where the beam impinges on it will no longer be in contact with the fluid and will be in contact with air instead. Air has a different index of refraction than the index of refraction of the fluid. Accordingly, upon reaching the diffraction surface, the reflected beam will no longer reflect out to the same point, and is reflected in such a manner that it impinges upon the sensor, which will then observe a reflected beam.